Why populations can’t be saved by a single breeding pair

I published this last week on The Conversation, and now reproducing it here for CB.com readers.

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Two days ago, the last male northern white rhino (Ceratotherium simum cottoni) died. His passing leaves two surviving members of his subspecies: both females who are unable to bear calves.

Even though it might not be quite the end of the northern white rhino because of the possibility of implanting frozen embryos in their southern cousins (C. simum simum), in practical terms, it nevertheless represents the end of a long decline for the subspecies. It also raises the question: how many individuals does a species need to persist?

No inbreeding, please

The Red List’s assessment criteria are based on the so-called “50/500 rule”. This states that to avoid inbreeding depression (the loss of “fitness” due to genetic problems), an effective population size of at least 50 individuals in a population is required.

To avoid eroding evolutionary potential (the ability of a population to evolve to cope with future environmental changes), an effective population of at least 500 is required.

The key here is that little qualifier “effective”. This refers to individuals who can breed with each other without causing inbreeding or loss of genetic diversity. A family unit, for example, might have only one or two reproductively effective members. But they would also need another, unrelated, family unit nearby for their offspring to reproduce with.

That means that the number of effective individuals is lower than the total population. On average, the ratio is about 0.1 to 0.2; that is, one effective individual (genetically speaking) for every five to ten members of the population.

This also assumes that the breeding pairs are matching up based on an optimal genetic basis – what geneticists call an “idealised population”.

In a perfect world, a breeding pair of animals would be completely unrelated and would have no chance of producing babies with any genetic defects caused by inbreeding. However, real populations rarely behave like this, so some pairs have a certain amount of relatedness. As the population gets smaller, the chance of breeding with a relative increases, which leads to more frequent and severe inbreeding.

Repopulating the world after the apocalypse

So let’s do the maths. Fifty effective individuals – the IUCN standard for avoiding inbreeding – equals a total population of 250 to 500. This means that, in a hypothetical apocalypse, humanity would need a lot more than a handful of survivors to repopulate effectively.

However, to retain evolutionary potential – to remain genetically flexible and diverse – the IUCN criteria suggest we would need at least 500 effective individuals. That requires a population of 2,500 to 5,000.

Some preliminary results emerging from ongoing research at the Centre of Excellence for Australian Biodiversity and Heritage appear to confirm this. Using both ancient DNA techniques and palaeo-demographic models, we have estimates of a minimum effective population size for Aboriginal Australians when they first appeared of about 250. This means at least several thousand had to arrive around the same time to manage to colonise the entire continent successfully.

Of course, not every species has the same ratio of effective to total population size, and not all populations necessarily need 5,000 individuals to survive. But without being able to measure the true ratio for a specific population, it helps to default to the average situation.

The idea that 50 individuals is enough to avoid inbreeding depression comes largely from laboratory populations that probably do not describe the situation for populations living in wild environments.

In species as varied as houseflies and pinkfairies, populations substantially greater than 50 individuals still succumb to inbreeding depression. So, in many cases, 50 effective individuals is in fact too low to ensure no inbreeding depression occurs. It may be that 100 effective individuals is closer to the true minimum, without even considering how populations respond to evolutionary challenges.

So, sensational analogies about the apocalypse aside, do human beings follow the same rule? We aren’t entirely sure, but evidence suggests that most species in vastly different groups roughly follow the same trend.

An emerging rule of thumb is that when a population starts to dip below several thousand individuals, it has a high likelihood of going extinct.

3 responses

4042018

Craig Morley(05:32:50) :

As for inbreeding depression issues, we know that islands have large inbred populations and species manage to survive so this this isn’t always true. Note that the “mythical 50/500 rule” is just a mathematical construct and would not work on small islands. While it would be ideal having large unrelated populations, inbreeding, while not desirable, is :possibly” better than extinction. Just think of the Chatham Island Black Robins where they managed to survive when down to only 1 female, and a few males so technically it is possible. They are now thriving nicely.

Dear Dr. Bradshaw,
As usual, very interesting this last post on your blog. While reading it, this question comes up to my mind: “and, invasive species?” at least some of them seem to escape to the inbreeding depression problem…
Thank you in advance,
Leire

However, it is only superficially a paradox because of the simple truth that most (> 99%) of all introductions (whether accidental or deliberate) fail. In other words, our perception is biased by the rather well-publicised ‘successful’ invasions.

Also, many successful invasions are successful because they involve multiple introductions (invasion success = number introduced × number of introductions), with each potentially contribution genetic diversity to the new population. So yes, successful invasions can occur, but in the grand scheme of things they’re rare and often do not necessarily result in inbreeding depression.

Another issue is that even when inbreeding depression occurs, it might not be enough to reduce vital rates to the point where the population growth rate (r) becomes negative, so the population can still grow. However, the resulting population might still lack the genetic diversity to withstand future environmental change. Remember – most introductions caused by humans are fairly recent. We still do not know what the long-term fate of many of them will be.